Many of us in our club, the Philly Flyers, run FP40s and FP35s. Our basic system is a large venturi, one or two extra head gaskets, 10.5x4.5 apc, 22% (50/50) Powermaster 5%, free flowing tongue muffler. It's quite possible that the run-away you're experiencing is a function of vibration, frequently a problem with ARF Profiles. See the ARF section. I ran down the involved process of debugging a Primary Force ARF powered by an FP40. The soft nose caused inconsistent engine runs and run-aways. The engine behaving much like the ones described by Andrew. As we reduced vibration the engine ran better and more consistently. The engine now works extremely well. In this case an 11x5 Zinger wood prop also helped to tame the engine. At the Middlesex, NJ, contest last weekend I used the PF/FP40 in Intermediate. It had a crackling nasty 2x4 or 2x2 switch back in level flight powering up quickly when doing the tricks. The engine combo was as effective as any good running 40 at the contest including the plus $200 engines.
Jack Weston, Dan Banjok and Mike Palko (in the past, before he became ELECTRO-CUTE) have used and continue to use FP40s in competition. The combination of power and break is the equal of any non-piped engine we've seen.
Note: we set these engines by ear, not by tack. Sneaking up on the sweet needle opening click by click, going from a slightly over rich setting to one that works well in flight. Depending on the airplane, the engine may run richer or even slightly leaner on the ground than in the air. Allow the engine to heat up on the ground running for thirty seconds or so before making a final adjustment for flying. Observe engine as it goes through the pattern. Reset needle, if needed, on the basis of the flight performance. So, in other words, if a slight lean run on the ground meant a good flight setting, go with that. Or visa versa. Sometimes the ground setting and flight setting are very similar. Often they are not. I had a Twister that ran dead lean on the ground. After take off the prop would unload and the engine would settle into a crackling switch back break in level flight. Usually we run muffler pressure to a non-uniflo vent, which means the engine will lean out gradually, which is usually a positive, providing more boost for the high and overhead maneuvers. Uniflo can be problematic, depending upon the airframe. Some of our planes do quite well on uniflo. The needle setting will vary somewhat depending on temperature and air density, even varying during the day. Extreme heat usually means less O in the air making for a richer setting. High humidity also means less O. Denser cold air means a leaner run at the same needle opening. It's important to richen the mixture when this happens. Running an FP40 on the lean side is usually not the way to go. IC engines, even the big buck stunt specials, also need adjustment to suit air conditions. Final note, a good running FP usually has a harsher sound than the typical LA, Brodak or dedicated stunt engine.